The present invention relates to a magnetic disk inspection apparatus and more particularly, to a magnetic disk inspection apparatus wherein the peripheral speed of an inspecting portion of a rotating magnetic disk corresponding to an inspection head is approximately constant.
Disk media such as a magnetic disk which must have a high degree of flatness on their surface have been inspected by dedicated inspection apparatus referred to as a "glide tester". This inspection method will be explained with reference to FIG. 2. In the drawing, a head 5 floats at a certain height H due to an air flow produced by the rotation of a magnetic disk 1. This floating height H is determined by the relative speed between the disk 1 and the head 5. If any protuberance 15 higher than the floating height H exists on the disk surface, the protuberance 15 impinges against the head 5, whereas in the case of another protuberance 16 which is lower than the floating height H, the head 5 does not impinge against it but passes by. Therefore, if a piezosensor 17 is provided to the head 5, the sensor 17 can detect this impingement and can find any protuberances which are higher than the floating height H. In FIG. 2, symbols IMP and OMP represent innermost periphery and outermost periphery of the region to be inspected, respectively.
During the inspection operation where the magnetic disk is rejected because protuberance higher than the floating height H exists, a predetermined floating height H must be always kept irrespective of the position of the head over the disk surface, as shown in FIG. 3. In FIG. 3, symbol A1 represents the case where the floating height H is set to be great and A2 represents the case where it is set to be small. In order to keep the floating height H constant, the relative speed between the disk 1 and the head 5 or in other words, the peripheral speed, must be kept constant. Since the head 5 is moved in a radial direction by a carriage 6, the peripheral speed is kept constant by changing the rotating speed of a spindle 2 which rotates the disk 1, in accordance with the radial position of the head 5. When the head 5 exists at a position of a radius r (mm) as shown in FIG. 4, the rotating speed .omega. (rpm) of the spindle and the peripheral speed V (mm/sec) of the head 5 with respect to the disk 1 have the following relation: ##EQU1##
In other words, the rotating speed .omega. and the radius r are inversely proportional, and this relation is as shown in FIG. 5 wherein the rotating speed .omega. and the radius r are plotted on the ordinate and on the abscissa, respectively. If the relative speed V between the head 5 and the disk 1 is to be kept constant, the rotating speed becomes higher as the head moves toward the inner peripheral side. In FIG. 5, curve B1 represents the case where a peripheral speed is higher and B2 does the case where a peripheral speed is lower. To provide this rotating speed curve, it has been customary in the past to employ a method which prepares a conversion table into which numeric values of this curve are written beforehand, reads out one by one a rotating speed value corresponding to a carriage position from the conversion table with the movement of the carriage and applies this value to a spindle driving apparatus so as to control the peripheral speed constant.
In the method described above which determines the rotating speed from the conversion table, a plurality of conversion tables are necessary in accordance with a plurality of floating height values and the conversion table must be expanded in order to obtain higher conversion accuracy. Therefore, a large capacity memory is necessary for the conversion table. Another problem is that a control circuit for selecting the conversion tables and for the readout operation gets rather complicated.